Conventionally, in order to produce hydrogen by decomposing ammonia, it is necessary to allow a reaction of the following formula (I) to proceed in the presence of a ruthenium-based ammonia decomposition catalyst.NH3 3/2H2+½N2  (I)                ΔH298K=46.1 kJ/mol        
Since the reaction of the formula (I) is an endothermic reaction, in order to obtain a stable ammonia decomposition ratio, it is necessary to provide heat to a reaction system and set the reaction temperature to 350° C. or higher.
Therefore, in order to suppress a decrease in gas temperature due to the endothermic reaction, heat is supplied from the outside conventionally. However, by this method, the heat transfer rate is lower than the reaction rate, and therefore, in order to obtain a sufficient heat transfer rate, there is no other choice but to increase a heat transfer area and it is difficult to decrease the size of an apparatus.
A method in which exhaust gas of an engine or the like is used as a heat source for supplying heat from the outside is also contemplated, however, this method has a drawback that, in the case where the temperature of the heat source is 350° C. or lower, since the temperature of the heat source is lower than the temperature at which a catalyst works, and therefore, heat cannot be supplied and a predetermined amount of hydrogen cannot be produced.
As the heat source for supplying heat, other than the supply from the outside, there is a method in which as shown in the following formula (II), heat is generated by a catalytic reaction between ammonia and oxygen, and the generated heat is used.NH3+¾O2½N2+ 3/2H2O  (II)                ΔH298K=−315.1 kJ/mol        
When the reaction of the formula (I) and the reaction of the formula (II) are caused in the same reaction tube, the heat absorbed by the reaction of the formula (I) can be supplemented with the heat generated by the reaction of the formula (II). Further, the temperature of a catalyst layer can be controlled by controlling the amount of oxygen in the formula (II). For example, in the case where the temperature of the supply gas preheated by waste heat of engine exhaust gas through heat exchange varies, hydrogen can be stably produced.
As an ammonia oxidation catalyst to be used for allowing the reaction of the formula (II) to proceed, a platinum-based catalyst is generally used. For example, in Patent Literature 1, a multilayer ammonia oxidation catalyst containing a refractory metal oxide, a platinum layer provided on this refractory metal oxide, and a vanadia layer provided on the platinum is proposed.
However, the operating temperature of the catalyst is about 200° C., and the oxidation reaction cannot be allowed to proceed at a temperature of about 200° C. or lower, and therefore, it is necessary to increase the gas temperature to about 200° C. with an electric heater or the like.
In Patent Literature 2, an ammonia oxidation catalyst containing an oxide of at least one element selected from cerium and praseodymium, an oxide of at least one element selected from non-variable valency rare earth elements including yttrium, and cobalt oxide is proposed, and in Patent Literature 3, an ammonia oxidation catalyst, which contains filaments composed essentially of platinum, rhodium, and optionally palladium, and in which the filaments have a platinum coating is proposed. However, these catalysts also have the same problem as in Patent Literature 1.
PTL 1: JP-T-2007-504945, the term “JP-T” as used herein means a published Japanese translation of a PCT patent application
PTL 2: Japanese Patent No. 4,165,661
PTL 3: JP-A-63-72344